If the design says 45k and I add 40k, the boat should still float? Submarines have this figured out because they use it to travel. I might be getting rid of a 1000 lb generator so the water line would change, just thinking about how much.
Maybe I can ask this differently. A boat with 2 feet of freeboard and displacement of 10k lbs. If you add 5k lbs would the waterline be at 1 foot?
The two numbers that you are talking about are not related. Displacement refers to how much fluid is displayed if floated on a liquid medium. It is a direct correlation to the mass, of the floating object. With a fairly consistent gravitational pull by the Earth, that mass is convertible to weight, which is an expression of force.
For boats, displacement comes is two basic numbers, three here. One is the weight, or displacement of the boat when just hull and basic equipment, the design displacement. The second is the cruising displacement, the weight of the boat when fully outfitted and ready to sail. This should be close to what the travel lift scales weigh, minus what had been removed to take home. The third displacement is closer to what you're thinking of, the loaded displacement. I may have the exact terms wrong, but this is the capacity, in cargo, that the boat is certified to hold. It is not the displacement required to sink the boat.
As JamesG pointed out. A simple linear relationship like what you are asking about would occur only if the bottom of the boat is flat and the sides were plumb. Then, every foot of boat below the waterline would displace the same amount as every foot of boat above the waterline.
It is my guess, that the use of displacement for boats came about because it is easier to calculate the volume of boat design than the weight when you don't know the materials you are building it out of are consistent in their density. English oak doesn't have the same density as mahogany or pine, for example.
-Will (Dragonfly)